Solid polymer electrolyte fuel cells ("SPFCs") generate electricity through the oxidation of a conventional fuel such as hydrogen. The relatively simple design and long demonstrated life of SPFCS make them particularly suitable for both stationary and motive applications.
A single solid polymer electrolyte fuel cell arrangement comprises an ion exchange membrane separating two electrodes, i.e., an anode and a cathode. The anode and cathode typically are sheets of porous carbon fiber paper between which the membrane is interposed. The three-layer anode-membrane-cathode assembly, or membrane electrode assembly, is interposed between electrically conductive graphite separator plates. The graphite plates collect current, facilitate the access of the fuel and oxidant to the anode and cathode surfaces, respectively, and provide for the removal of water formed during the operation of the cell. A plurality of fuel cell assemblies are usually configured together to form an SPFC stack.
To prevent reactant gases from escaping into the atmosphere from between the separator plates in each fuel cell assembly, the portion of the ion exchange membrane extending beyond the electrochemically active region can be used as a gasket seal between the plates. More recent fuel cell designs attempt to conserve expensive membrane material by using gaskets made of elastomeric material around the edges of the graphite separator plates instead of using the membrane itself as a gasket. The membrane material does not extend substantially beyond the electrochemically active region in the more recent design. Instead, the gasket extends between the graphite separator plates along the outer periphery of the plates and around the manifold openings in the plates.
Conventional solid polymer fuel cells are temperature regulated by a cooling fluid circulation system. To maintain proper cell temperature, individual or small groups of fuel cell assemblies are interposed between rigid, electrically conductive plates which form a cooling jacket. A coolant fluid (usually water) is directed through the cooling jacket to absorb heat energy released by the electrochemical reaction within the fuel cell. The heat is transferred to the coolant fluid as a result of the thermal gradient between the reaction site and the coolant.
Typically, flow field grooves are molded or machined on the surfaces of the cooling jacket plates facing the fuel cell assemblies to accommodate cooling fluid distribution and heated fluid elimination. In most water-cooled fuel cell assemblies, the heated water exits through an exhaust manifold and is then used to humidify the incoming fuel and oxidant gases.
The flow field grooves in conventional cooling jacket designs direct the coolant in a "cross-plate" fashion. That is, the coolant essentially flows from one side of the plate to the other. After absorbing heat while passing through multiple flow channels, the warmed coolant is collected in an exhaust channel on one side of the plate and delivered to a coolant outlet or exhaust manifold. The exhaust channel is located along an edge of the cooling plate edge in close proximity to the gasket used to seal the plates.
This conventional cooling jacket design has several disadvantages. One disadvantage arises because the flow of heated cooling fluid at the coolant exit concentrates heat around the oxidant and coolant exhaust channels. This arrangement results in elevated temperatures near the coolant outlet, causing gasket sealing failures and subsequent reactant leaks in the seal area near the exhaust channel. Furthermore, gasket sealing failures can result in contamination of the fuel cell from external impurities, particularly in submersible applications, which can decrease the lifetime of the membrane and the fuel cell components generally.